Mechanically adaptive organic transistors for implantable electronics

Cutting-Edge Perovskite-Based Flexible Pressure Sensors Made Possible by Piezoelectric Innovation

In the area of flexible electronics, pressure sensors are a widely utilized variety of flexible electronics that are both indispensable and prevalent. The importance of pressure sensors in various fields is currently increasing, leading to the exploration of materials with unique structural and piezoelectric properties. Perovskite-based materials are ideal for use as flexible pressure sensors (FPSs) due to their flexibility, chemical composition, strain tolerance, high piezoelectric and piezoresistive properties, and potential integration with other technologies. This article presents a comprehensive study of perovskite-based materials used in FPSs and discusses their components, performance, and applications in detecting human movement, electronic skin, and wireless monitoring. This work also discusses challenges like material instability, durability, and toxicity, the limited widespread application due to environmental factors and toxicity concerns, and complex fabrication and future directions for perovskite-based FPSs, providing valuable insights for researchers in structural health monitoring, physical health monitoring, and industrial applications.

Softening implantable bioelectronics: Material designs, applications, and future directions

Implantable bioelectronics, integrated directly within the body, represent a potent biomedical solution for monitoring and treating a range of medical conditions, including chronic diseases, neural disorders, and cardiac conditions, through personalized medical interventions. Nevertheless, contemporary implantable bioelectronics rely heavily on rigid materials (e.g., inorganic materials and metals), leading to inflammatory responses and tissue damage due to a mechanical mismatch with biological tissues. Recently, soft electronics with mechanical properties comparable to those of biological tissues have been introduced to alleviate fatal immune responses and improve tissue conformity. Despite their myriad advantages, substantial challenges persist in surgical handling and precise positioning due to their high compliance. To surmount these obstacles, softening implantable bioelectronics has garnered significant attention as it embraces the benefits of both rigid and soft bioelectronics. These devices are rigid for easy standalone implantation, transitioning to a soft state in vivo in response to environmental stimuli, which effectively overcomes functional/biological problems inherent in the static mechanical properties of conventional implants. This article reviews recent research and development in softening materials and designs for implantable bioelectronics. Examples featuring tissue-penetrating and conformal softening devices highlight the promising potential of these approaches in biomedical applications. A concluding section delves into current challenges and outlines future directions for softening implantable device technologies, underscoring their pivotal role in propelling the evolution of next-generation bioelectronics.

A temperature-responsive intravenous needle that irreversibly softens on insertion

The high stiffness of intravenous needles can cause tissue injury and increase the risk of transmission of blood-borne pathogens through accidental needlesticks. Here we describe the development and performance of an intravenous needle whose stiffness and shape depend on body temperature. The needle is sufficiently stiff for insertion into soft tissue yet becomes irreversibly flexible after insertion, adapting to the shape of the blood vessel and reducing the risk of needlestick injury on removal, as we show in vein phantoms and ex vivo porcine tissue. In mice, the needles had similar fluid-delivery performance and caused substantially less inflammation than commercial devices for intravenous access of similar size. We also show that an intravenous needle integrated with a thin-film temperature sensor can monitor core body temperature in mice and detect fluid leakage in porcine tissue ex vivo. Temperature-responsive intravenous needles may improve patient care.

Electrochemically actuated microelectrodes for minimally invasive peripheral nerve interfaces

Electrode arrays that interface with peripheral nerves are used in the diagnosis and treatment of neurological disorders; however, they require complex placement surgeries that carry a high risk of nerve injury. Here we leverage recent advances in soft robotic actuators and flexible electronics to develop highly conformable nerve cuffs that combine electrochemically driven conducting-polymer-based soft actuators with low-impedance microelectrodes. Driven with applied voltages as small as a few hundreds of millivolts, these cuffs allow active grasping or wrapping around delicate nerves. We validate this technology using in vivo rat models, showing that the cuffs form and maintain a self-closing and reliable bioelectronic interface with the sciatic nerve of rats without the use of surgical sutures or glues. This seamless integration of soft electrochemical actuators with neurotechnology offers a path towards minimally invasive intraoperative monitoring of nerve activity and high-quality bioelectronic interfaces.

Synthesis of shape-programmable elastomer for a bioresorbable, wireless nerve stimulator

Materials that have the ability to manipulate shapes in response to stimuli such as heat, light, humidity and magnetism offer a means for versatile, sophisticated functions in soft robotics or biomedical implants, while such a reactive transformation has certain drawbacks including high operating temperatures, inherent rigidity and biological hazard. Herein, we introduce biodegradable, self-adhesive, shape-transformable poly (L-lactide-co-ε-caprolactone) (BSS-PLCL) that can be triggered via thermal stimulation near physiological temperature (∼38 °C). Chemical inspections confirm the fundamental properties of the synthetic materials in diverse aspects, and study on mechanical and biochemical characteristics validates exceptional stretchability up to 800 % and tunable dissolution behaviors under biological conditions. The integration of the functional polymer with a bioresorbable electronic system highlights potential for a wide range of biomedical applications.

Skin-inspired, sensory robots for electronic implants

Drawing inspiration from cohesive integration of skeletal muscles and sensory skins in vertebrate animals, we present a design strategy of soft robots, primarily consisting of an electronic skin (e-skin) and an artificial muscle. These robots integrate multifunctional sensing and on-demand actuation into a biocompatible platform using an in-situ solution-based method. They feature biomimetic designs that enable adaptive motions and stress-free contact with tissues, supported by a battery-free wireless module for untethered operation. Demonstrations range from a robotic cuff for detecting blood pressure, to a robotic gripper for tracking bladder volume, an ingestible robot for pH sensing and on-site drug delivery, and a robotic patch for quantifying cardiac function and delivering electrotherapy, highlighting the application versatilities and potentials of the bio-inspired soft robots. Our designs establish a universal strategy with a broad range of sensing and responsive materials, to form integrated soft robots for medical technology and beyond.

Analysis of a Flexible Photoconductor, Manufactured with Organic Semiconductor Films

This work presents the evaluation of the electrical behavior of a flexible photoconductor with a planar heterojunction architecture made up of organic semiconductor films deposited by high vacuum evaporation. The heterojunction was characterized in its morphology and mechanical properties by scanning electron microscopy and atomic force microscopy. The electrical characterization was carried out through the approximations of ohmic and SCLC (Space-Charge Limited Current) behaviors using experimental J-V (current density-voltage) curves at different voltages and under different light conditions. The optimization of the photoconductor was carried out through annealing and accelerated lighting processes. With these treatments, the Knoop Hardness of the flexible photoconductor has reached a value of 8 with a tensile strength of 5.7 MPa. The ohmic and SCLC approximations demonstrate that the unannealed device has an ohmic behavior, whereas the annealed device has an SCLC behavior, and after the optimization process, an ohmic behavior and a maximum current density of 0.34 mA/mm2 were obtained under blue light. The approximations of the device's electron mobility (μn) and free carrier density (n0) were performed under different light conditions, and the electrical activation energy and electrical gap were obtained for the flexible organic device, resulting in appropriate properties for these applications.

Submicron-Thickness Ultraflexible Organic Light-Emitting Diodes via a Photoregulated Stripping Strategy

With the rapid advances in imperceptible and epidermal electronics, the research on ultraflexible organic light-emitting diodes (OLEDs) has become increasingly significant, owing to their excellent flexibility and conformability to the human body. It is highly desirable to develop submicrometer-thick ultraflexible OLEDs to enable the devices to seamlessly conform to the surface of arbitrary-shaped objects and still function properly. However, it remains a huge challenge for currently reported OLEDs due to the lack of an appropriate stripping strategy. Here, for the first time, we develop a facile photoregulated stripping strategy for the fabrication of high-performance ultraflexible OLEDs with submicron thickness. Under ultraviolet (UV) irradiation, the surface adhesion force of the ultrathin photopolymer membrane can be adjusted from 16.9 to 5.1 N/m, thereby effectively controlling the laminating and detaching process. Based on this strategy, the resultant device thickness is as low as 0.821 μm, which is the lowest record among flexible OLEDs reported to date. More remarkably, excellent electrical properties with a maximum current efficiency (CE) of 62.5 cd/A, an external quantum efficiency (EQE) of 17.8%, and a low turn-on voltage of 2.5 V are realized, which are superior to almost all of the reported ultraflexible OLEDs with thicknesses below 10 μm. Based on versatile ultraflexible OLEDs, all-organic and skin-mounted displays are successfully realized by employing a conformable organic thin-film transistor (OTFT) as the driver. This work offers a feasible strategy for advancing OLEDs from flexible to ultraflexible, showing significant application potential in future epidermal electronics and conformal displays.

Water-Responsive 3D Electronics for Smart Biological Interfaces

Three-dimensional (3D) electronic systems with their potential for enhanced functionalities often require complex fabrication processes. This paper presents a water-based, stimuli-responsive approach for creating self-assembled 3D electronic systems, particularly suited for biorelated applications. We utilize laser scribing to programmatically shape a water-responsive bilayer, resulting in smart 3D electronic substrates. Control over the deformation direction, actuation time, and surface curvature of rolling structures is achieved by adjusting laser-scribing parameters, as validated through experiments and numerical simulations. Additionally, self-locking structures maintain the integrity of the 3D systems. This methodology enables the implementation of spiral twining electrodes for electrophysiological signal monitoring in plants. Furthermore, the integration of self-rolling electrodes onto peripheral nerves in a rodent model allows for stimulation and recording of in vivo neural activities with excellent biocompatibility. These innovations provide viable paths to next-generation 3D biointegrated electronic systems for life science studies and medical applications.

Konjac glucomannan-based hydrogels with health-promoting effects for potential edible electronics applications: A mini-review

Konjac glucomannan (KGM), a dietary fiber hydrocolloid polysaccharide isolated from Amorphophallus konjac tubers, has potential applications in various fields. However, the use of KGM-based hydrogels has mainly focused on the food, biomedical, and water treatment industries. KGM possesses several health benefits and could be a promising candidate for use in edible electronics. This paper presents the first review of KGM-based hydrogels as edible electronics and their potential health benefits. The paper initially focuses on the health-promoting effects of KGM-based hydrogels, such as prebiotic effects, antiobesity, antioxidant, and antibacterial properties. Then, it discusses the feasible design strategies for KGM-based hydrogels as edible electronics, considering their flexibility, mechanical properties, response to stimuli, degradability aspects, their role as electronic device components, and the retention period of the devices. Finally, this review outlines future directions for developing KGM-based hydrogels for use in edible electronics.

Fine-Tuning the Performance of Ultraflexible Organic Complementary Circuits on a Single Substrate via a Nanoscale Interfacial Photochemical Reaction

Flexible electronics has paved the way toward the development of next-generation wearable and implantable healthcare devices, including multimodal sensors. Integrating flexible circuits with transducers on a single substrate is desirable for processing vital signals. However, the trade-off between low power consumption and high operating speed is a major bottleneck. Organic thin-film transistors (OTFTs) are suitable for developing flexible circuits owing to their intrinsic flexibility and compatibility with the printing process. We used a photoreactive insulating polymer poly((±)endo,exo-bicyclo[2.2.1]hept-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE) to modulate the power consumption and operating speed of ultraflexible organic circuits fabricated on a single substrate. The turn-on voltage (V _on) of the p- and n-type OTFTs was controlled through a nanoscale interfacial photochemical reaction. The time-of-flight secondary ion mass spectrometry revealed the preferential occurrence of the PNDPE photochemical reaction in the vicinity of the semiconductor-dielectric interface. The power consumption and operating speed of the ultraflexible complementary inverters were tuned by a factor of 6 and 4, respectively. The minimum static power consumption was 30 ± 9 pW at transient and 4 ± 1 pW at standby. Furthermore, within the tuning range of the operating speed and at a supply voltage above 2.5 V, the minimum stage delay time was of the order of hundreds of microseconds. We demonstrated electromyogram measurements to emphasize the advantage of the nanoscale interfacial photochemical reaction. Our study suggests that a nanoscale interfacial photochemical reaction can be employed to develop imperceptible and wearable multimodal sensors with organic signal processing circuits that exhibit low power consumption.

Electrothermally Actuated Semitransparent Shape Memory Polymer Composite with Application as a Wearable Touch Sensor

A semitransparent shape memory polymer (SMP):silver nanowire (AgNW) composite is demonstrated to be capable of low-temperature actuation, thus making it attractive for wearable electronics applications that require intimate contact with the human body. We demonstrate that the SMP:AgNW composite has tunable electrical and optical transparency through variation of the AgNW loading and that the AgNW loading did not significantly change the mechanical behavior of the SMP. The SMP composite is also capable of electrical actuation through Joule heating, where applying a 4 V bias across the AgNWs resulted in full shape recovery. The SMP was found to have high strain sensitivity at both small (<1%) and large (over 10%) applied strain. The SMP could sense strains as low as 0.6% with a gauge factor of 8.2. The SMP composite was then utilized as a touch sensor, able to sense and differentiate tapping and pressing. Finally, the composite was applied as a wearable ring that was thermally actuated to conformably fit onto a finger as a touch sensor. The ring sensor was able to sense finger tapping, pressing, and bending with high signal-to-noise ratios. These results demonstrate that SMP:AgNW composites are a promising design approach for application in wearable electronics.

Materials and design strategies for stretchable electroluminescent devices

Stretchable displays have recently received increasing attention as input and/or output interfaces for next-generation human-friendly electronic systems. Stretchable electroluminescent (EL) devices are a core component of stretchable displays, and they can be classified into two types, structurally stretchable EL devices and intrinsically stretchable EL devices, according to the mechanism for achieving their stretchability. We herein present recent advances in materials and design strategies for stretchable EL devices. First, stretchable devices based on ultrathin EL devices are introduced. Ultrathin EL devices are mechanically flexible like thin paper, and they can become stretchable through various structural engineering methods, such as inducing a buckled structure, employing interconnects with stretchable geometries, and applying origami/kirigami techniques. Secondly, intrinsically stretchable EL devices can be fabricated by using inherently stretchable electronic materials. For example, light-emitting electrochemical cells and EL devices with a simpler structure using alternating current have been developed. Furthermore, novel stretchable semiconductor materials have been presented for the development of intrinsically stretchable light-emitting diodes. After discussing these two types of stretchable EL devices, we briefly discuss applications of deformable EL devices and conclude the review.

Shape and stiffness memory ionogels with programmable pressure-resistance response

Flexible pressure sensors usually require functional materials with both mechanical compliance and appropriate electrical performance. Most sensors based on materials with limited compressibility can hardly balance between high sensitivity and broad pressure range. Here, we prepare a heterophasic ionogel with shape and stiffness memory for adaptive pressure sensors. By combining the microstructure alignment for stiffness changing and shape memory micro-inclusions for stiffness fixing, the heterophasic ionogels reveal tunable compressibility. This controllable pressure-deformation property of the ionogels results in the pressure sensors' programmable pressure-resistance behavior with tunable pressure ranges, varied detection limits, and good resolution at high pressure. Broad pressure ranges to 220 and 380 kPa, and tunable detection limit from 120 to 330 and 950 Pa are realized by the stiffness memory ionogel sensors. Adaptive detection is also brought out to monitor tiny pressure changes at low stiffness and distinguish different human motions at high stiffness. Using shape and stiffness memory materials in pressure sensors is a general design to achieve programmable performance for more complex application scenarios.

Review on Microscale Sensors with 3D Engineered Structures: Fabrication and Applications

The intelligence of modern technologies relies on perceptual systems based on microscale sensors. However, because of the traditional top-down fabrication approaches performed on planar silicon wafers, a large proportion of existing microscale sensors have 2D structures, which severely restricts their sensing capabilities. To overcome these restrictions, over the past few decades, increasing efforts have been devoted to developing new fabrication methods for microscale sensors with 3D engineered structures, from bulk chemical etching and 3D printing to molding and stress-induced assembly. Herein, the authors systematically review these fabrication methods based on the applications of the resulting 3D sensors and discuss their advantages compared to their 2D counterparts. This is followed by a perspective on the remaining challenges and possible opportunities.

Recent Progress in Materials Chemistry to Advance Flexible Bioelectronics in Medicine

Designing bioelectronic devices that seamlessly integrate with the human body is a technological pursuit of great importance. Bioelectronic medical devices that reliably and chronically interface with the body can advance neuroscience, health monitoring, diagnostics, and therapeutics. Recent major efforts focus on investigating strategies to fabricate flexible, stretchable, and soft electronic devices, and advances in materials chemistry have emerged as fundamental to the creation of the next generation of bioelectronics. This review summarizes contemporary advances and forthcoming technical challenges related to three principal components of bioelectronic devices: i) substrates and structural materials, ii) barrier and encapsulation materials, and iii) conductive materials. Through notable illustrations from the literature, integration and device fabrication strategies and associated challenges for each material class are highlighted.

Recent Advances in High-Mobility and High-Stretchability Organic Field-Effect Transistors: From Materials, Devices to Applications

Stretchable organic field-effect transistors (OFETs) are one of the essential building blocks for next-generation wearable electronics due to the high stretchability of OFET well matching with the large deformation of human skin. In recent years, some significant progress of stretchable OFETs have already been made via the strategies of stretchable molecular design and geometry engineering. However, the main opportunity and challenge of stretchable OFETs is still to simultaneously improve their stretchability and mobility. This review covers the recent advances in the research of stretchable OFETs with high mobility. First, the core stretchable materials are summarized, including organic semiconductors, electrodes, dielectrics, and substrates. Second, the materials and healing mechanism of self-healing OFET are summarized in detail. Subsequently, their different configurations and the potential applications are summarized. Finally, an outlook of future research directions and challenges in this area is presented.

Transistor-Based Biomolecule Sensors: Recent Technological Advancements and Future Prospects

Transistor-based sensors have been widely recognized to be highly sensitive and reliable for point-of-care/bed-side diagnosis. In this line, a range of cutting-edge technologies has been generated to elevate the role of transistors for biomolecule detection. Detection of a wide range of clinical biomarkers has been reported using various configurations of transistors. The inordinate sensitivity of transistors to the field-effect imparts high sensitivity toward wide range of biomolecules. This overview has gleaned the present achievements with the technological advancements using high performance transistor-based sensors. This review encloses transistors incorporated with a variety of functional nanomaterials and organic elements for their excellence in selectivity and sensitivity. In addition, the technological advancements in fabrication of these microdevices or nanodevices and functionalization of the sensing elements have also been discussed. The technological gap in the realization of sensors in transistor platforms and the resulted scope for research has been discussed. Finally, foreseen technological advancements and future research perspectives are described.

Flexible Electrodes for In Vivo and In Vitro Electrophysiological Signal Recording

A variety of electrophysiological signals (electrocardiography, electromyography, electroencephalography, etc.) are generated during the physiological activities of human bodies, which can be collected by electrodes and thus provide critical insights into health status or facilitate fundamental scientific research. The long-term stable and high-quality recording of electrophysiological signals is the premise for their further applications, leading to demands for flexible electrodes with similar mechanical modulus and minimized irritation to human bodies. This review summarizes the latest advances in flexible electrodes for the acquisition of various electrophysiological signals. First, the concept of electrophysiological signals and the characteristics of different subcategory signals are introduced. Second, the invasive and noninvasive methods are reviewed for electrophysiological signal recording with a highlight on the design of flexible electrodes, followed by a discussion on their material selection. Subsequently, the applications of the electrophysiological signal acquisition in pathological diagnosis and restoration of body functions are discussed, showing the advantages of flexible electrodes. Finally, the main challenges and opportunities in this field are discussed. It is believed that the further exploration of materials for flexible electrodes and the combination of multidisciplinary technologies will boost the applications of flexible electrodes for medical diagnosis and human-machine interface.

4D Printing of shape-memory polymeric scaffolds for adaptive biomedical implantation

4D printing has shown great potential in a variety of biomedical applications due to the adaptability and minimal invasiveness of fabricated devices. However, commonly employed shape memory polymers (SMPs) possess undesirable transition temperatures (T_transs), leading to complications in implantation operations. Herein, we demonstrate 4D printing of a new SMP named poly(glycerol dodecanoate) acrylate (PGDA) with a T_trans in a range of 20 °C - 37 °C making it appropriate for shape programming at room temperature and then shape deployment within the human body. In addition, the material possesses suitable rheological properties to allow for the fabrication of a variety of delicate 3D structures such as "triangular star", "six-petal flower", "honeycomb", "tube", tilted "truncated hollow cones", as well as overhanging "bridge", "cage", and "mesh". The printed 3D structures show shape memory properties including a large fixity ratio of 100% at 20 °C, a large recovery ratio of 98% at 37 °C, a stable cyclability of > 100 times, and a fast recovery speed of 0.4 s at 37 °C. Moreover, the Young's moduli of the printed structures can be decreased by 5 times due to the phase transition of PGDA, which is compatible with biological tissues. Finally, in vitro stenting and in vivo vascular grafting demonstrated the geometrical and mechanical adaptivity of the printed constructs for biomedical implantation. This newly developed PGDA SMP based 4D printing technology has the potential to pave a new route to the fabrication of shape memory scaffolds for personalized biomedical applications.

A Review of Shape Memory Polymers and Composites: Mechanisms, Materials, and Applications

Over the past decades, interest in shape memory polymers (SMPs) has persisted, and immense efforts have been dedicated to developing SMPs and their multifunctional composites. As a class of stimuli-responsive polymers, SMPs can return to their initial shape from a programmed temporary shape under external stimuli, such as light, heat, magnetism, and electricity. The introduction of functional materials and nanostructures results in shape memory polymer composites (SMPCs) with large recoverable deformation, enhanced mechanical properties, and controllable remote actuation. Because of these unique features, SMPCs have a broad application prospect in many fields covering aerospace engineering, biomedical devices, flexible electronics, soft robotics, shape memory arrays, and 4D printing. Herein, a comprehensive analysis of the shape recovery mechanisms, multifunctionality, applications, and recent advances in SMPs and SMPCs is presented. Specifically, the combination of functional, reversible, multiple, and controllable shape recovery processes is discussed. Further, established products from such materials are highlighted. Finally, potential directions for the future advancement of SMPs are proposed.

Tissue-Adaptive Materials with Independently Regulated Modulus and Transition Temperature

The ability of living species to transition between rigid and flexible shapes represents one of their survival mechanisms, which has been adopted by various human technologies. Such transition is especially desired in medical devices as rigidity facilitates the implantation process, while flexibility and softness favor biocompatibility with surrounding tissue. Traditional thermoplastics cannot match soft tissue mechanics, while gels leach into the body and alter their properties over time. Here, a single-component system with an unprecedented drop of Young's modulus by up to six orders of magnitude from the GPa to kPa level at a controlled temperature within 28-43 °C is demonstrated. This approach is based on brush-like polymer networks with crystallizable side chains, e.g., poly(valerolactone), affording independent control of melting temperature and Young's modulus by concurrently altering side chain length and crosslink density. Softening down to the tissue level at the physiological temperature allows the design of tissue-adaptive implants that can be inserted as rigid devices followed by matching the surrounding tissue mechanics at body temperature. This transition also enables thermally triggered release of embedded drugs for anti-inflammatory treatment.

Facile Approach to Fabricating Stretchable Organic Transistors with Laser-Patterned Ag Nanowire Electrodes

Stretchable electronics are poised to revolutionize personal healthcare and robotics, where they enable distributed and conformal sensors. Transistors are fundamental building blocks of electronics, and there is a need to produce stretchable transistors using low-cost and scalable fabrication techniques. Here, we introduce a facile fabrication approach using laser patterning and transfer printing to achieve high-performance, solution-processed intrinsically stretchable organic thin-film transistors (OTFTs). The device consists of Ag nanowire (NW) electrodes, where the source and drain electrodes are patterned using laser ablation. The Ag NWs are then partially embedded in a poly(dimethylsiloxane) (PDMS) matrix. The electrodes are combined with a PDMS dielectric and polymer semiconductor, where the layers are individually transfer printed to complete the OTFT. Two polymer semiconductors, DPP-DTT and DPP-4T, are considered and show stable operation under the cyclic strain of 20 and 40%, respectively. The OTFTs maintain electrical performance by adopting a buckled structure after the first stretch-release cycle. The conformability and stretchability of the OTFT is also demonstrated by operating the transistor while adhered to a finger being flexed. The ability to pattern highly conductive Ag NW networks using laser ablation to pattern electrodes as well as interconnects provides a simple strategy to produce complex stretchable OTFT-based circuits.

Active and Deformable Organic Electronic Devices based on Conductive Shape Memory Polyimide

Smart, deformable, and transparent electrodes are a significant part of flexible optoelectronic devices. In this work, a novel approach to making highly transparent, smooth, and conductive shape memory polyimide hybrids has been proposed. Colorless shape memory polyimide (CSMPI) with high optical transparency and high heat resistance is served as the substrate for flexible electronic devices for the first time. A hybrid (Au/Ag) metal grid electrode embedded in CSMPI (BMG/CSMPI) is first fabricated via self-cracking template and solution-coating, the advantages of which include ultrasmooth surface, superior mechanical flexibility and durability, strong surface adhesion, and excellent chemical stability due to the unique embedded hybrid structure. The resulting white polymer light emitting diodes (WPLEDs) based on BMG/CSMPI with shape memory effect are active and deformable, and are converted from 2D device into 3D devices depending on its variable stiffness characteristics. The deformed 3D devices could actively recover to the original shape upon heating. Furthermore, ultrathin and flexible 3D optoelectronic devices fabricated using shape memory polymers can promote the development of advanced optoelectronic applications in the future.

Injectable Biomedical Devices for Sensing and Stimulating Internal Body Organs

The rapid pace of progress in implantable electronics driven by novel technology has created devices with unconventional designs and features to reduce invasiveness and establish new sensing and stimulating techniques. Among the designs, injectable forms of biomedical electronics are explored for accurate and safe targeting of deep-seated body organs. Here, the classes of biomedical electronics and tools that have high aspect ratio structures designed to be injected or inserted into internal organs for minimally invasive monitoring and therapy are reviewed. Compared with devices in bulky or planar formats, the long shaft-like forms of implantable devices are easily placed in the organs with minimized outward protrusions via injection or insertion processes. Adding flexibility to the devices also enables effortless insertions through complex biological cavities, such as the cochlea, and enhances chronic reliability by complying with natural body movements, such as the heartbeat. Diverse types of such injectable implants developed for different organs are reviewed and the electronic, optoelectronic, piezoelectric, and microfluidic devices that enable stimulations and measurements of site-specific regions in the body are discussed. Noninvasive penetration strategies to deliver the miniscule devices are also considered. Finally, the challenges and future directions associated with deep body biomedical electronics are explained.

Conformable Hybrid Systems for Implantable Bioelectronic Interfaces

Conformable bioelectronic systems are promising tools that may aid the understanding of diseases, alleviate pathological symptoms such as chronic pain, heart arrhythmia, and dysfunctions, and assist in reversing conditions such as deafness, blindness, and paralysis. Combining reduced invasiveness with advanced electronic functions, hybrid bioelectronic systems have evolved tremendously in the last decade, pushed by progress in materials science, micro- and nanofabrication, system assembly and packaging, and biomedical engineering. Hybrid integration refers here to a technological approach to embed within mechanically compliant carrier substrates electronic components and circuits prepared with traditional electronic materials. This combination leverages mechanical and electronic performance of polymer substrates and device materials, respectively, and offers many opportunities for man-made systems to communicate with the body with unmet precision. However, trade-offs between materials selection, manufacturing processes, resolution, electrical function, mechanical integrity, biointegration, and reliability should be considered. Herein, prominent trends in manufacturing conformable hybrid systems are analyzed and key design, function, and validation principles are outlined together with the remaining challenges to produce reliable conformable, hybrid bioelectronic systems.

3D Self-Assembled Microelectronic Devices: Concepts, Materials, Applications

Modern microelectronic systems and their components are essentially 3D devices that have become smaller and lighter in order to improve performance and reduce costs. To maintain this trend, novel materials and technologies are required that provide more structural freedom in 3D over conventional microelectronics, as well as easier parallel fabrication routes while maintaining compatability with existing manufacturing methods. Self-assembly of initially planar membranes into complex 3D architectures offers a wealth of opportunities to accommodate thin-film microelectronic functionalities in devices and systems possessing improved performance and higher integration density. Existing work in this field, with a focus on components constructed from 3D self-assembly, is reviewed, and an outlook on their application potential in tomorrow's microelectronics world is provided.

Multiscale Soft-Hard Interface Design for Flexible Hybrid Electronics

Emerging next-generation soft electronics will require versatile properties functioning under mechanical compliance, which will involve the use of different types of materials. As a result, control over material interfaces (particularly soft/hard interfaces) has become crucial and is now attracting intensive worldwide research efforts. A series of material and structural interface designs has been devised to improve interfacial adhesion, preventing failure of electromechanical properties under mechanical deformation. Herein, different soft/hard interface design strategies at multiple length scales in the context of flexible hybrid electronics are reviewed. The crucial role of soft ligands and/or polymers in controlling the morphologies of active nanomaterials and stabilizing them is discussed, with a focus on understanding the soft/hard interface at the atomic/molecular scale. Larger nanoscopic and microscopic levels are also discussed, to scrutinize viable intrinsic and extrinsic interfacial designs with the purpose of promoting adhesion, stretchability, and durability. Furthermore, the macroscopic device/human interface as it relates to real-world applications is analyzed. Finally, a perspective on the current challenges and future opportunities in the development of truly seamlessly integrated soft wearable electronic systems is presented.

Flexible Hybrid Electronics for Digital Healthcare

Recent advances in material innovation and structural design provide routes to flexible hybrid electronics that can combine the high-performance electrical properties of conventional wafer-based electronics with the ability to be stretched, bent, and twisted to arbitrary shapes, revolutionizing the transformation of traditional healthcare to digital healthcare. Here, material innovation and structural design for the preparation of flexible hybrid electronics are reviewed, a brief chronology of these advances is given, and biomedical applications in bioelectrical monitoring and stimulation, optical monitoring and treatment, acoustic imitation and monitoring, bionic touch, and body-fluid testing are described. In conclusion, some remarks on the challenges for future research of flexible hybrid electronics are presented.

Exploring the Critical Thickness of Organic Semiconductor Layer for Enhanced Piezoresistive Sensitivity in Field-Effect Transistor Sensors

Organic semiconductors (OSCs) are promising transducer materials when applied in organic field-effect transistors (OFETs) taking advantage of their electrical properties which highly depend on the morphology of the semiconducting film. In this work, the effects of OSC thickness (ranging from 5 to 15 nm) on the piezoresistive sensitivity of a high-performance p-type organic semiconductor, namely dinaphtho [2,3-b:2,3-f] thieno [3,2–b] thiophene (DNTT), were investigated. Critical thickness of 6 nm thin film DNTT, thickness corresponding to the appearance of charge carrier percolation paths in the material, was demonstrated to be highly sensitive to mechanical strain. Gauge factors (GFs) of 42 ± 5 and −31 ± 6 were measured from the variation of output currents of 6 nm thick DNTT-based OFETs engineered on top of polymer cantilevers in response to compressive and tensile strain, respectively. The relationship between the morphologies of the different thin films and their corresponding piezoresistive sensitivities was discussed.

In Situ Reversible Tuning from Pinned to Roll-Down Superhydrophobic States on a Thermal-Responsive Shape Memory Polymer by a Silver Nanowire Film

Shape memory polymer (SMP) surfaces with tunable wettability have attracted extensive attention due to their widespread applications. However, there have been rare reports on in situ tuning wettability with SMP materials. In this paper, we reported a kind of distinct superhydrophobic SMP microconed surface on the silver nanowire (AgNW) film to achieve in situ reversible transition between pinned and roll-down states. The mechanism is taking advantage of the in situ heating functionality of the silver nanowire film by voltage, which provides the transition energy for SMP to achieve the fixation and recovery of temporary shape. It is noteworthy that the reversible transition could be repeated many times (>100 cycles), and we quantitatively investigate the shape memory ability of microcones with varied height and space under different applied voltages. These results show that the tilted microcones could recover its original upright state under a small voltage (4-11 V) in a short time, and the shortest recovery time is about 0.5 min under an applied voltage of ∼10 V. Finally, we utilize SMP microcone arrays with tunable wettability to realize lossless droplet transportation, and the tilted microconed surface also achieves liquid unidirectional transport due to its anisotropic water adhesion force. The robust microconed SMP surface with reversible morphology transitions will have far-ranging applications including droplet manipulation, reprogrammable fog harvesting, and so on.

Machine-knitted washable sensor array textile for precise epidermal physiological signal monitoring

Wearable textile electronics are highly desirable for realizing personalized health management. However, most reported textile electronics can either periodically target a single physiological signal or miss the explicit details of the signals, leading to a partial health assessment. Furthermore, textiles with excellent property and comfort still remain a challenge. Here, we report a triboelectric all-textile sensor array with high pressure sensitivity and comfort. It exhibits the pressure sensitivity (7.84 mV Pa^-1), fast response time (20 ms), stability (>100,000 cycles), wide working frequency bandwidth (up to 20 Hz), and machine washability (>40 washes). The fabricated TATSAs were stitched into different parts of clothes to monitor the arterial pulse waves and respiratory signals simultaneously. We further developed a health monitoring system for long-term and noninvasive assessment of cardiovascular disease and sleep apnea syndrome, which exhibits great advancement for quantitative analysis of some chronic diseases.

Interface Engineering in Organic Field-Effect Transistors: Principles, Applications, and Perspectives

Heterogeneous interfaces that are ubiquitous in optoelectronic devices play a key role in the device performance and have led to the prosperity of today's microelectronics. Interface engineering provides an effective and promising approach to enhancing the device performance of organic field-effect transistors (OFETs) and even developing new functions. In fact, researchers from different disciplines have devoted considerable attention to this concept, which has started to evolve from simple improvement of the device performance to sophisticated construction of novel functionalities, indicating great potential for further applications in broad areas ranging from integrated circuits and energy conversion to catalysis and chemical/biological sensors. In this review article, we provide a timely and comprehensive overview of current efficient approaches developed for building various delicate functional interfaces in OFETs, including interfaces within the semiconductor layers, semiconductor/electrode interfaces, semiconductor/dielectric interfaces, and semiconductor/environment interfaces. We also highlight the major contributions and new concepts of integrating molecular functionalities into electrical circuits, which have been neglected in most previous reviews. This review will provide a fundamental understanding of the interplay between the molecular structure, assembly, and emergent functions at the molecular level and consequently offer novel insights into designing a new generation of multifunctional integrated circuits and sensors toward practical applications.

Transfer Printing of Electronic Functions on Arbitrary Complex Surfaces

Transfer printing of electronic functions on arbitrary surfaces is essential for next-generation applications of skin-attachable electronics, wearable sensors, and implantable/medical devices. For transfer printing of electronic functions on multidimensional surfaces, such as curved regions of the skin and different objects, various strategies have been devised based on the materials and structural design of electronic components and transfer stamps, such as ultrathin membranes or in-plane structures of electronic components, soft interfacial glues or adhesives between devices and surfaces, and smart transfer adhesives with bioinspired micro/nanostructures. These techniques enable high conformity of adhesion, mechanical robustness, and high compliance of electronic devices on arbitrary surfaces under mechanical deformation. In this Perspective, we provide an overview of recent transfer printing techniques and discuss their advantages and challenges. In addition, we report a recently developed transfer printing technique based on bioinspired smart adhesives with reversible adhesion, which enables compliant electronics on various arbitrary complex surfaces without performance degradation, providing solutions for various technical challenges remaining in transfer printing. Finally, we present potential applications of transfer printing and future perspectives for this emerging field.

Direct writing of 3D conjugated polymer micro/nanostructures for organic electronics and bioelectronics

3D direct writing and meniscus-guided pen writing methods, which are capable of fabricating 3D micro/nanostructures from soluble π-conjugated polymers (CPs) and CP precursors, and recent advances in these techniques are addressed in this review.

Complex multiphase organohydrogels with programmable mechanics toward adaptive soft-matter machines

Many biological organisms can tune their mechanical properties to adapt to environments in multistable modes, but the current synthetic materials, with bistable states, have a limited ability to alter mechanical stiffness. Here, we constructed programmable organohydrogels with multistable mechanical states by an on-demand modular assembly of noneutectic phase transition components inside microrganogel inclusions. The resultant multiphase organohydrogel exhibits precisely controllable thermo-induced stepwise switching (i.e., triple, quadruple, and quintuple switching) mechanics and a self-healing property. The organohydrogel was introduced into the design of soft-matter machines, yielding a soft gripper with adaptive grasping through stiffness matching with various objects under pneumatic-thermal hybrid actuation. Meanwhile, a programmable adhesion of octopus-inspired robotic tentacles on a wide range of surface morphologies was realized. These results demonstrated the applicability of these organohydrogels in lifelike soft robotics in unconstructed and human body environments.

Stimulus Responsive 3D Assembly for Spatially Resolved Bifunctional Sensors

3D electronic/optoelectronic devices have shown great potentials for various applications due to their unique properties inherited not only from functional materials, but also from 3D architectures. Although a variety of fabrication methods including mechanically guided assembly have been reported, the resulting 3D devices show no stimuli-responsive functions or are not free standing, thereby limiting their applications. Herein, the stimulus responsive assembly of complex 3D structures driven by temperature-responsive hydrogels is demonstrated for applications in 3D multifunctional sensors. The assembly driving force, compressive buckling, arises from the volume shrinkage of the responsive hydrogel substrates when they are heated above the lower critical solution temperature. Driven by the compressive buckling force, the 2D-formed membrane materials, which are pre-defined and selectively bonded to the substrates, are then assembled to 3D structures. They include "tent," "tower," "two-floor pavilion," "dome," "basket," and "nested-cages" with delicate geometries. Moreover, the demonstrated 3D bifunctional sensors based on laser induced graphene show capability of spatially resolved tactile sensing and temperature sensing. These multifunctional 3D sensors would open new applications in soft robotics, bioelectronics, micro-electromechanical systems, and others.

Remotely Triggered Assembly of 3D Mesostructures Through Shape-Memory Effects

3D structures that incorporate high-performance electronic materials and allow for remote, on-demand 3D shape reconfiguration are of interest for applications that range from ingestible medical devices and microrobotics to tunable optoelectronics. Here, materials and design approaches are introduced for assembly of such systems via controlled mechanical buckling of 2D precursors built on shape-memory polymer (SMP) substrates. The temporary shape fixing and recovery of SMPs, governed by thermomechanical loading, provide deterministic control over the assembly and reconfiguration processes, including a range of mechanical manipulations facilitated by the elastic and highly stretchable properties of the materials. Experimental demonstrations include 3D mesostructures of various geometries and length scales, as well as 3D aquatic platforms that can change trajectories and release small objects on demand. The results create many opportunities for advanced, programmable 3D microsystem technologies.

Carbon Nanotube/Conducting Polymer Hybrid Nanofibers as Novel Organic Bioelectronic Interfaces for Efficient Removal of Protein-Bound Uremic Toxins

Protein-bound uremic toxins (PBUTs) can cause noxious effects in patients suffering from renal failure as a result of inhibiting the transport of proteins and inducing their structural modification. They are difficult to remove through standard hemodialysis (HD) treatment. Herein, we report an organic bioelectronic HD device system for the effective removal of PBUTs through electrically triggered dissociation of protein-toxin complexes. To prepare this system, we employed electrospinning to fabricate electrically conductive quaternary composite nanofiber mats-comprising multiwalled carbon nanotubes (MWCNTs), poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), poly(ethylene oxide) (PEO), and (3-glycidyloxypropyl)trimethoxysilane (GOPS)-on conventional polyethersulfone (PES) dialysis membranes. These composite nanofiber platforms exhibited (i) long-term water resistance (due to cross-linking among PSS, PEO, and GOPS), (ii) high adhesion strength on the PES membrane (due to GOPS functioning as an adhesion promoter), (iii) enhanced electrical properties [due to the MWCNTs and PEDOT:PSS promoting effective electrical stimulation (ES) operation in devices containing bioelectronic interfaces (BEI)], and (iv) good anticoagulant ability and negligible hemolysis of red blood cells. We employed this organic BEI electronic system as a novel single-membrane HD device to study the removal efficiency of four kinds of uremic toxins [p-cresol (PC), indoxyl sulfate, and hippuric acid as PBUTs; creatinine as a non-PBUT] as well as the effects of ES on lowering the protein binding ratio. Our organic BEI devices provided a high rate of removal of PC with low protein loss after 4 h of a simulated dialysis process. It also functioned with low complement activation, low contact activation levels, and lower amounts of platelet adsorption, suggesting great suitability for use in developing next-generation bioelectronic medicines for HD.

Bioinspired Adhesive Architectures: From Skin Patch to Integrated Bioelectronics

The attachment phenomena of various hierarchical architectures found in nature have extensively drawn attention for developing highly biocompatible adhesive on skin or wet inner organs without any chemical glue. Structural adhesive systems have become important to address the issues of human-machine interactions by smart outer/inner organ-attachable devices for diagnosis and therapy. Here, advances in designs of biologically inspired adhesive architectures are reviewed in terms of distinct structural properties, attachment mechanisms to biosurfaces by physical interactions, and noteworthy fabrication methods. Recent demonstrations of bioinspired adhesive architectures as adhesive layers for medical applications from skin patches to multifunctional bioelectronics are presented. To conclude, current challenges and prospects on potential applications are also briefly discussed.

Electrical Properties of Thiol-ene-based Shape Memory Polymers Intended for Flexible Electronics

Thiol-ene/acrylate-based shape memory polymers (SMPs) with tunable mechanical and thermomechanical properties are promising substrate materials for flexible electronics applications. These UV-curable polymer compositions can easily be polymerized onto pre-fabricated electronic components and can be molded into desired geometries to provide a shape-changing behavior or a tunable softness. Alternatively, SMPs may be prepared as a flat substrate, and electronic circuitry may be built directly on top by thin film processing technologies. Whichever way the final structure is produced, the operation of electronic circuits will be influenced by the electrical and mechanical properties of the underlying (and sometimes also encapsulating) SMP substrate. Here, we present electronic properties, such as permittivity and resistivity of a typical SMP composition that has a low glass transition temperature (between 40 and 60 °C dependent on the curing process) in different thermomechanical states of polymer. We fabricated parallel plate capacitors from a previously reported SMP composition (fully softening (FS)-SMP) using two different curing processes, and then we determined the electrical properties of relative permittivity and resistivity below and above the glass transition temperature. Our data shows that the curing process influenced the electrical permittivity, but not the electrical resistivity. Corona-Kelvin metrology evaluated the quality of the surface of FS-SMP spun on the wafer. Overall, FS-SMP demonstrates resistivity appropriate for use as an insulating material.

Softening gold for elastronics

Gold, one of the noble metals, has played a significant role in human society throughout history. Gold's excellent electrical, optical and chemical properties make the element indispensable in maintaining a prosperous modern electronics industry. In the emerging field of stretchable electronics (elastronics), the main challenge is how to utilize these excellent material properties under various mechanical deformations. This review covers the recent progress in developing "softening" gold chemistry for various applications in elastronics. We systematically present material synthesis and design principles, applications, and challenges and opportunities ahead.

Bio-Integrated Wearable Systems: A Comprehensive Review

Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of "bio-integrated" technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in following sections. The subsequent content highlights the most advanced biosensors, classified according to their ability to capture biophysical, biochemical, and environmental information. Additional sections feature schemes for electrically powering these sensors and strategies for achieving fully integrated, wireless systems. The review concludes with an overview of key remaining challenges and a summary of opportunities where advances in materials chemistry will be critically important for continued progress.